The invention relates to a method and an apparatus for testing and analyzing piezoelectric transducers, and monitoring sensor devices employing piezoelectric transducers. These devices are used in the detection of changes in stress, pressure, temperature, acceleration, acoustic emissions, and infra and ultra sound motions. Piezoelectric transducers emit an electrical signal when subjected to variations of stress, pressure, temperature, acceleration, etc. In most applications, piezoelectric transducers are bonded or fixed to a surface. The characteristics of the interface between the crystal and the surface can distort and attenuate the signal emitted, by exerting additional stress. As the size of the piezoceramic transducer is reduced, the signal becomes smaller and more difficult to detect and analyze, while the effect of the interface is substantially increased. In some cases the initial stress generated through the interface is so great that the transducer properties are lost. It is desirable to have a procedure to analyze the transducer and the effects of an interface in order to determine if the transducer is functioning satisfactorily. Many applications of piezoelectric transducers involve detection or monitoring of very small variations in stress, pressure, temperature, and acceleration. Under current systems of analysis these small variations cannot be detected because the signal to noise ratio of the transducer becomes critical. It is desirable to have a procedure that permits easy analysis of these very small variations.
Piezoelectric transducers are used for monitoring of machinery, processes or structures, and are installed for long term monitoring of machinery, processes or structures. In such long term monitoring, it is desirable to have a procedure to evaluate various stresses without putting the machinery into operation, particularly where the transducer is employed for detecting internal fractures and fissures in metal components.
Common methods used to monitor and analyze piezoelectric transducer signal properties generally involve physical mechanical shock fixtures, ultrasound exciters, air pressure shocks, electrical shock and evaluation of the transducer as an electronic component using electric waves. All of these methods affect either the motor effect, or the generator effect of the piezoelectric transducer, or both, by applying a constant or permanent initial stress to the transducer. This does not allow the transducer free to emit its own signal and renders it insensitive to very small external stress which is the subject of analysis. Consequently these testing methods do not allow measurement or analysis of the transducer without the presence of the effects generated by the testing method, do not allow measurement or analysis of the stress on the transducer from the fixture or bonding interface, and do not allow measurement of the system stress or effect on the transducer. None of these testing methods allow evaluation of resonance frequencies of the objects in contact with the transducer when the interface fixture or bonding stress is too great. Finally in the case of electric pulse test methods and apparatus, the condenser properties of many piezoelectric materials prevent analysis of the signal because a component of the shock wave is retained by the piezoelctric transducer.